The present invention relates to a power distribution control apparatus for a four-wheel drive vehicle.
Four-wheel drive vehicles include part-time four-wheel drive vehicles, full-time four-wheel drive vehicles, and standby four-wheel drive vehicles. A part-time four-wheel drive vehicle switches between four-wheel drive and two-wheel drive as needed. A full-time four-wheel drive vehicle constantly drives the four wheels. A standby four-wheel drive vehicle automatically switches between four-wheel drive and two-wheel drive.
A typical standby four-wheel drive vehicle has a transaxle including a transmission and a transfer. The transaxle transmits power of the engine to the front wheels through a pair of front axles. The transaxle transmits power of the engine to a power coupling device through a propeller shaft. The coupling device is coupled to a rear differential with a drive pinion shaft. The rear differential is coupled to the rear wheels with a pair of rear axles.
A typical coupling device includes an electromagnetic clutch mechanism of a multi-plate wet type. The clutch mechanism includes several clutch disks. When electricity is supplied to an electromagnetic coil in the clutch mechanism, the clutch disks are engaged with one another, which permits the power of the engine to be transmitted to the rear differential through the propeller shaft. The power transmitted to the rear differential is transmitted to the rear wheels through the rear axles. The frictional force among the clutch disks is determined by the amount of current supplied to the electromagnetic coil. The greater the frictional force is, the greater the power transmitted to the rear wheels is. In a standby four-wheel drive vehicle, a power distribution control apparatus for controlling the coupling device controls the frictional force of the electromagnetic clutch, thereby selecting one of four-wheel drive and two-wheel drive. The control apparatus also determines the power distribution between the front wheels and the rear wheels in the four-wheel drive.
The control apparatus controls the frictional force of the electromagnetic clutch mechanism based on a limited-slip differential torque map, which is prepared in advance. The torque map is a map table defining the value of current supplied to the electromagnetic coil to obtain a target frictional force. The target frictional force is set to optimize the power distribution for the driving state.
Specifically, the difference between the average rotation speed of the front wheels and the average rotation speed of the rear wheels, the opening degree of the throttle valve, and the vehicle speed are detected by corresponding sensors, and are used for obtaining the driving state. The control apparatus computes a command value based on the parameters detected by the sensors by referring to the map table, and controls the current to the electromagnetic coil based on the command value. Accordingly, the control apparatus controls the frictional force in the electromagnetic clutch mechanism to be a target value to optimize the power distribution for the current driving state. This permits the vehicle to run in a stable manner.
If the front wheels or rear wheels of a standby four-wheel drive vehicle is caught in sand, the caught wheels race due to lack of traction between the wheels and the ground. In other words, the vehicle is stuck in the sand. This increases the differential rotation speed between the front wheels and the rear wheels. If the differential rotation speed is above a predetermined threshold value over a predetermined period, the control apparatus determines that there is not sufficient traction, and performs a maximum torque control to maximize the frictional force of the electromagnetic clutch mechanism. The maximum torque control increases the traction to a sufficient level and permits the vehicle to escape the stuck condition.
However, in some cases, maximum torque control cannot increase the traction to a sufficient level and the vehicle cannot escape the stuck condition. In such a case, the control apparatus performs the maximum torque control for an extended period. An extended maximum torque control causes the clutch disks to be heated by friction, and excessive heat can damage the clutch disks.
Therefore, if the differential rotation speed between the front wheels and the rear wheels is not fall below the predetermined threshold value when a predetermined period has elapsed from when the maximum torque control is started, the vehicle is switched to two-wheel drive control to set the frictional force in the clutch mechanism to zero, thereby preventing the clutch disks from being damaged. This temporarily cools the heated clutch mechanism, and the clutch disks are prevented from being damaged.
In some cases, the vehicle remains in the two-wheel drive control for some time after the clutch disks of the clutch mechanism are sufficiently cooled and does not immediately return to the maximum torque control. As a result, the power distribution is not optimized. The vehicle therefore cannot smoothly escape the stuck condition.
In the maximum torque control for escaping the stuck condition, the front wheels are directly coupled to the rear wheels with the propeller shaft. In this case, a tight corner braking phenomenon occurs if the vehicle turns a corner due to unabsorbed difference between the speeds of the front wheels and the rear wheels.
Accordingly, it is an objective of the present invention to provide a power distribution control apparatus that prevents clutch disks from being damaged by heat, accurately distributes power, and guarantees a stable driving.
To achieve the above objects, one aspect of the present invention provides a power distribution control apparatus for a four-wheel drive vehicle. The vehicle includes a pair of front wheels and a pair of rear wheels, the wheels being driven by an engine, and a coupling device that changes the power distribution to the front wheels and the rear wheels. The front wheels or the rear wheels are first wheels that are coupled to the engine without the coupling device in between. The other wheels are second wheels that are coupled to the engine through the coupling device. The apparatus determines a command value that corresponds to the driving state of the vehicle. The frictional force of the coupling device determines the power distribution ratio to the first wheels and the second wheels. The frictional force of the coupling device is changed based on the command value. The apparatus includes detecting means, estimating means and determining means. The detecting means detects the differential rotation speed between the rotation speed of the front wheels and the rotation speed of the rear wheels. The estimating means for estimating the exothermic energy generated in the coupling device based on the product of the differential rotation speed and the command value. The selecting means determines an optimum drive mode for the four-wheel drive vehicle based on the estimated exothermic energy and for selecting map data corresponding to the determined drive mode. The determining means determines the command value that corresponds to the driving state by using the map data.
Another aspect of the present invention provides a method for controlling a power distribution control apparatus for a four-wheel drive vehicle. The vehicle includes a pair of front wheels and a pair of rear wheels, the wheels being driven by an engine, and a coupling device that changes the power distribution to the front wheels and the rear wheels. The front wheels or the rear wheels are first wheels that are coupled to the engine without the coupling device in between. The other wheels are second wheels that are coupled to the engine through the coupling device. The apparatus determines a command value that corresponds to the driving state of the vehicle. The frictional force of the coupling device determines the power distribution ratio to the first wheels and the second wheels. The frictional force of the coupling device is changed based on the command value. The method includes detecting the differential rotation speed between the rotation speed of the front wheels and the rotation speed of the rear wheels; estimating the exothermic energy generated in the coupling device based on the product of the differential rotation speed and the command value; determining an optimum drive mode for the four-wheel drive vehicle based on the estimated exothermic energy and selecting map data corresponding to the determined drive mode; and determining the command value that corresponds to the driving state by using the map data.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.